Thrust limiter for boom-mounted rotor

ABSTRACT

An electrical signal is received via a boom where a rotor is attached to the boom and the received electrical signal is received while the rotor outputs a thrust. It is decided whether to decrease the thrust which is output by the rotor, including by analyzing the received electrical signal. In response to deciding to decrease the thrust which is output by the rotor, the thrust which is output by the rotor is decreased.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 15/916,736 entitled THRUST LIMITER FOR BOOM-MOUNTED ROTOR filedMar. 9, 2018 which is incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

New types of ultralight aircraft are being developed which weigh muchless than conventional airplanes. To meet the stringent weightrequirements, many of the parts are hollow and/or are built of materialwhich (if subjected to enough strain and/or bending) can break. Forexample, hollow aircraft parts made of carbon composites are attractivefor their light weight. With these types of parts (e.g., hollow, made ofcarbon fiber, etc.), new techniques to monitor strain on aircraft partsand respond accordingly are needed. Furthermore, it would be desirableif these new techniques were extremely lightweight.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1 is a flowchart illustrating an embodiment of a process to limitthe thrust, if needed, of a boom-mounted rotor to prevent damage to theboom.

FIG. 2A is a diagram illustrating an embodiment of an unbent boom viawhich an electrical signal is received.

FIG. 2B is a diagram illustrating an embodiment of a bent boom via whichan electrical signal is received.

FIG. 3A is a diagram showing an angled-view embodiment of a float andtwo connected booms.

FIG. 3B is a diagram illustrating a top view embodiment of a multicopterwhich monitors four booms for bending or stress and adjusts thrust ifneeded.

FIG. 4A is a diagram illustrating an embodiment of a connector at a testpoint which uses a screw inserted into the composite.

FIG. 4B is a diagram illustrating an embodiment of a connector at a testpoint which uses a connector placed next to exposed composite withadhesive applied over the connector.

FIG. 4C is a diagram illustrating an embodiment of a connector at a testpoint which uses a conductive layer.

FIG. 5 is a graph illustrating an embodiment of different responsefunctions depending upon the state of the boom.

FIG. 6 is a flowchart illustrating an embodiment of a process to limitthe thrust, if needed, of a boom-mounted rotor to prevent damage to theboom where calibration is performed.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

Various embodiments of a technique to detect stress or strain on a boom(e.g., due to bending, where at least some of the bending is due to thethrust produced by a rotor attached to the distal end of the boom) anddecrease the thrust produced by a rotor (which is attached to the distalend of the boom) are described herein. As will be described in moredetail below, this technique (generally speaking) uses an electricalsignal which has passed through the boom to decide when the boom isexperiencing too much bending and/or stress (and therefore could breakor be damaged) and (in response) the thrust output by a boom-mountedrotor is reduced. This, in turn, reduced the bending and/or strainexperienced by the boom. First, an example process is described. Then,an exemplary aircraft which uses this technique is described.

FIG. 1 is a flowchart illustrating an embodiment of a process to limitthe thrust, if needed, of a boom-mounted rotor to prevent damage to theboom. In some aircraft embodiments, booms of the aircraft are hollowwith relatively small cross sectional areas. As a result, such booms aremore susceptible to bending which in turn puts stress on the boom. Boomsare often made of a composite material and so if the boom is bent toomuch, the boom could break or otherwise be damaged. In some embodiments,the process of FIG. 1 is performed across booms in a multicopter orother aircraft where monitoring and resulting thrust limiting isdesired. In some embodiments, the process of FIG. 1 is performedcontinuously, since the stress or bending experienced by a boom willvary over the course of a flight.

At 100, an electrical signal is received via a boom, wherein a rotor isattached to the boom and the received electrical signal is receivedwhile the rotor outputs a thrust. For example, a signal generator mayinject or otherwise insert an electrical signal into the boom at one endof the boom (e.g., near a proximal end where the boom is attached to afuselage or near a distal end where the rotor is attached). For clarity,this electrical signal is sometimes referred to as the injectedelectrical signal (e.g., to differentiate from the received electricalsignal).

A signal receiver attached to the other end of the boom (e.g., oppositethe signal generator) receives a received electrical signal which haspassed or otherwise traveled through the boom from the signal generatorto the signal receiver; for clarity, this signal is sometimes referredto as the received electrical signal.

In some embodiments, the injected electrical signal and receivedelectrical signal are direct current (DC) signals generated by aconstant current source or a constant voltage source (e.g., with nofrequency or phase). In some embodiments, the injected electrical signaland received electrical signal are alternating current (AC) signalswhich have a phase, frequency, and amplitude.

At 102, it is decided whether to decrease the thrust which is output bythe rotor, including by analyzing the received electrical signal. Theboom will exhibit different electrical properties when it is bent and/orstressed (e.g., due in part to the thrust generated by the rotor)compared to when the boom is unbent and/or unstressed. For example, theresistance (for DC signals) or impedance (for AC signals) through theboom could increase (or decrease) as the boom is bent and/or stressedmore and more.

In one example of step 102, the received electrical signal is a DCsignal and the current (or voltage) level of the received electricalsignal is analyzed to detect an increase (or decrease) in the current(or voltage) level. For example, there may be an increase (or decrease)in the current (or voltage) level compared to some baseline levelcorresponding to an unbent and/or unstressed boom when the rotor is off.

In another example, the received electrical signal is an AC signal andthe phase, frequency, and/or amplitude of the received electrical signalis analyzed to detect an increase (or decrease) compared to somebaseline phase, frequency, and/or amplitude (e.g., when the rotor is offand the boom is unbent and/or unstressed).

A naïve approach to deciding when to limit a rotor's thrust is to use afixed thrust ceiling for a particular rotor that the rotor is notpermitted to exceed. However, the conditions under which a boom is bentand/or stressed too much can vary greatly and therefore using a fixedthrust ceiling will not properly detect (e.g., for all conditions and/orstates) when the thrust should be reduced to prevent damage to the boom.For example, suppose that stress testing is being performed on anaircraft and the aircraft is “locked down” so that it cannot move evenwhen the rotor(s) are rotating. Depending upon a variety of stateinformation, including environmental state information (e.g., thetemperature, whether water has permeated the boom (e.g., some compositematerials use adhesives which can absorb water)) and/or intrinsic stateinformation (e.g., the age of the boom), various electrical and/orphysical properties of the materials which make up the boom will varyand therefore how much thrust is “too much” will also vary. This meansthat a given amount of thrust can be safe for the boom with one set ofenvironmental and/or intrinsic states (e.g., the temperature, the amountof water permeation, the age of the boom, etc.) while that same amountof thrust can be potentially damaging with another set of environmentaland/or intrinsic states.

Other types of state information which may affect when the boom is beingbent and/or stressed “too much” includes flight-related stateinformation. For example, suppose that an aircraft is no longer “lockeddown” and is free to fly away. Even if a rotor is generating a steady orconstant amount of thrust, the amount of stress and/or bendingexperienced by a boom will be different under different flightconditions (e.g., when the aircraft is banking or turning at a high rateof speed versus hovering in air at a fixed position). More formally,flight-related state information may include position (including angularposition such as attitude, yaw, pitch, roll, etc.), velocity,acceleration, etc. Using an electrical signal to decide when to reducethe thrust output by a boom-mounted rotor can pick up on all of thesefactors (e.g., environmental state information, intrinsic stateinformation, flight-related state information, etc.) and is thereforebetter at deciding when to reduce thrust compared to using a fixedthrust ceiling.

At 104, in response to deciding to decrease the thrust which is outputby the rotor, the thrust which is output by the rotor is decreased. Asdescribed above, too much bending and/or stress on the boom could causethe boom to break. Since at least some of the bending and/or stress isdue to the thrust from the rotor, the rotor's thrust is decreased toreduce the stress and/or bending experienced by the boom. In oneexample, the weight of the pilot is known. With a lighter pilot, lessbending and/or stress on the booms is expected compared to when there isa heavier pilot. In some embodiments, if the amount of bending and/orstress (e.g., represented or manifested in the electrical signal) isgreater than expected (e.g., for a lighter pilot), the thrust isdecreased because it is assumed that something is wrong (e.g., given thepilot's relatively light weight and the expected behavior or responsegiven that weight).

In some embodiments, the thrust output by other rotors is increased inorder to compensate for the reduced thrust at step 104. For example,suppose that the aircraft is a multicopter where there are multiplerotors. Some of the rotors may be connected to more structurally robustparts of the aircraft (e.g., a float with a larger cross-sectional areaand/or which is reinforced). As a result, those rotors can output morethrust without putting additional stress on the boom in question and insome embodiments thrust from those rotors is increased to compensate forthe reduced thrust from the rotor attached to the boom.

It may be helpful to consider the process of FIG. 1 using an examplesystem. The following figures show an example of a rotor-mounted boomwhich performs thrust limitation using an electrical signal through theboom.

FIG. 2A is a diagram illustrating an embodiment of an unbent boom viawhich an electrical signal is received. In this example, the rotor (200a) is not rotating and is therefore not producing any (e.g., downward)thrust. The rotor (200 a) is attached to one end of a boom (202 a) andsince there is no thrust output by the rotor, the boom is unbent in itsnatural or “at rest” position with no stress or strain on the boom.

Signal generator (204 a) generates an injected electrical signal (208 a)which is injected or otherwise inserted into one end of the unbent boom(202 a). The injected electrical signal passes through the unbent boom,for example following one or more paths with the least electricalresistance or impedance. For example, the boom may include compositematerials which have sufficient electrical properties or characteristicsin order to conduct an electrical signal (i.e., the boom is anelectrical conductor as opposed to an electrical insulator). Forsimplicity and ease of explanation, suppose that the unbent boom (202 a)shown here has a resistance of R_(unbent) (alternatively, an impedanceof I_(unbent) for AC signals).

A signal receiver (206 a) captures or otherwise receives the electricalsignals which have passed through the unbent boom (202 a). It is notedthat the signal generator (204 a) and signal receiver (206 a) aresubstantially at or near the ends of the boom so that the electricalsignal passes through most of the boom.

In some embodiments, a boom is hollow (e.g., ultralight aircraft havevery stringent weight requirements and hollowing out the boom reducesweight). In such embodiments where the boom is hollow, the electricalsignal would merely pass through the available material or crosssection. For example, the electrical signal is not expected to passthrough the air in the hollow core of a boom since air is assumed tohave a higher electrical resistance or impedance than the materialswhich are used to construct the boom.

The following figure shows an example when the boom is bent and/orstressed.

FIG. 2B is a diagram illustrating an embodiment of a bent boom via whichan electrical signal is received. In this example, the rotor (200 b) isrotating and producing a downward thrust which is strong enough to bendthe boom. For example, although not shown here, the left end of the boommay be attached to a fuselage so that when the rotor at the right end ofthe boom produces enough thrust, the boom bends upward slightly.

With the boom bent as shown, the signal generator (204 b) producesanother injected electrical signal (208 b). For simplicity and ease ofexplanation, suppose that this second injected electrical signal (208 b)is identical to the first injected electrical signal (208 a) in FIG. 2A.The bending of the boom causes the electrical properties of the boom tochange, which produces differences in the received electrical signal(210 b) which is received by the signal receiver (206 b).

For example, suppose that the resistance (impedance) of the bent boom(202 b) is now R_(bent) where R_(bent)≠R_(unbent). If the injectedelectrical signal is a constant voltage signal (e.g., where the signalgenerator adjusts the current to maintain a constant voltage), thendifferent current values or levels may be observed in the first receivedelectrical signal (210 a) versus the second received electrical signal(210 b) due to the different resistances (i.e., R_(bent)≠R_(unbent))(alternatively, impedances). Or, if the injected electrical signal is aconstant current signal (e.g., where the signal generator adjusts thevoltage to maintain a constant current), different voltages may beobserved in the first received electrical signal (210 a) compared to thesecond received electrical signal (210 b). Constant current and voltagesignals may more generally be referred to as DC signals.

In some embodiments, an AC signal is passed through the boom. Forexample, the injected signal may be some sinusoidal signal with afrequency, amplitude, and phase. Due to different electrical propertiesin the unbent boom (202 a) versus the bent boom (202 b), there may bedifferences in the one or more properties in the received electricalsignal when the boom is bent versus when the boom is unbent (e.g., thefrequency shift or spreads from a single frequency to multiplefrequencies, the amplitude changes, the phase changes, etc.). In variousembodiments, an analysis of a received electrical signal which includesan AC signal may be performed in the time domain or the frequencydomain. For example, in the time domain, a phase shift or change may beeasier to detect. In the frequency domain, a frequency shift orfrequency spreading may be easier to detect. Amplitude changes may bedetected in either the time domain or frequency domain.

One benefit to this technique is that it is very light weight. The boom,which acts as the electrical conductor between the test insertion pointand the test extraction point is already there and adds no additionalweight. In some embodiments, the monitoring and any subsequent thrustreduction is done by a flight controller and/or microprocessors (e.g.,which the aircraft already had). In that case, the only new parts wouldbe any new copper wires and/or connectors to carry the electricalsignals between the flight controller and the test sites where they areinjected into the boom or extracted from the boom. The exemplarymulticopter shown here is an ultralight weight aircraft and so alightweight technique is very attractive for such an aircraft.

It is noted that the electrical properties associated with a boom (e.g.,unbent or bent) would be the same if the positions or locations of theelectrical transmitter (204 a/204 b) and electrical receiver (206 a/206b) were switched. To put it another way, the positions of the electricaltransmitter and electrical receiver are merely exemplary and in someembodiments, the electrical transmitter is located at the same end ofthe boom as the rotor. As will be described in an example below, in someembodiments, both the signal generator and signal receiver are locatedcloser to the proximal end of the boom since most of the stress orbending is experienced near the proximal end of the boom (e.g., region212).

The following figure shows an exemplary multicopter and test pointswhere the injected electrical signal and received electrical signal areinjected into and extracted from the boom, respectively.

FIG. 3A is a diagram showing an angled-view embodiment of a float andtwo connected booms. In the example shown, the exemplary multicopter iscapable of landing on and taking off from water. Float (300) and acounterpart float on the other side of the multicopter (not shown)provide the necessary buoyancy to float. As shown here, the float isalso used to store the five batteries which power the five rotors onthis side of the multicopter.

The five rotors on this side of the multicopter are arranged with threeinner rotors (302 a-302 c) connected to the float (300). The two outerrotors (304 and 306) are connected to the distal ends of the front boom(308) and back boom (310), respectively. Although not shown here, therotors on the other side of the multicopter are similarly arranged withthree inner rotors and two outer rotors.

From the view shown, it is apparent that the booms (308 and 310) couldbend and therefore be damaged since the booms have a relatively smallcross sectional area. If the stress due to bending is not sufficientlymonitored and corrected for, the bending could cause the front boom(308) or back boom (310) to break. In contrast, the relatively largecross sectional area of the float (300) makes the float morestructurally robust and therefore less susceptible to bending and/orstress. Therefore, in this example, the booms (308 and 310) aremonitored using an electrical signal, whereas the float (300) is notmonitored.

The following figure shows a top view of the exemplary multicopter withexemplary test points.

FIG. 3B is a diagram illustrating a top view embodiment of a multicopterwhich monitors four booms for bending or stress and adjusts thrust ifneeded. In the example shown, a different view of the same multicopterfrom FIG. 3A is shown. In this view, the exemplary multicopter includesfour booms (320, 322, 324, and 326) where each of the booms is attachedto a(n outer) rotor at their distal ends and to the fuselage (328) attheir proximal ends.

In this example, each of the booms (320, 322, 324, and 326) isindependently monitored and the thrust output by a corresponding rotor(350, 352, 354, and 356, respectively) is similarly adjusted in anindependent manner (e.g., because one boom could have too much bendingor stress while another boom is fine). If too much bending or stress isdetected across a given boom, the corresponding rotor at the end of thatboom is slowed down so that less thrust is produced.

The following table describes the related booms, test points (e.g.,where the injected electrical signal is injected into the boom and wherethe received electrical signal is extracted or otherwise obtained fromthe boom, or vice versa), and adjusted rotors, if needed. For example,if too much bending or stress is detected along the front left boom(320) using test points 330 and 332, the outer front left rotor (350) isslowed down so that less thrust is produced (e.g., to reduce the bendingand/or stress experienced by the front left boom).

TABLE 1 Related booms, test points, and affected rotors from FIG. 3B.Rotor with Reduced Thrust Boom Test Points (If Needed) Front Left Boom(320) 330 and 332 Outer Front Left Rotor (350) Front Right Boom (322)334 and 336 Outer Front Right Rotor (352) Back Left Boom (324) 338 and340 Outer Back Left Rotor (354) Back Right Boom (326) 342 and 344 OuterBack Right Rotor (356)

Between the distal end and proximal end of each boom, there is anattachment point where that boom is attached to either the left float(360) or right float (362). The floats (as seen in FIG. 3A) and fuselage(328) have a relatively large cross sectional area which makes themrelatively robust (e.g., structurally). As a result, the parts of thebooms that lie between the fuselage (328) and floats (360 and 362)experience relatively little bending and/or stress. Rather, most of thebending and/or stress in the booms occurs between the floats and thedistal ends of the booms (e.g., where the outer rotors are located) andmore specifically closer to the boom rather than the outer rotor.

As a result of where the bending and/or stress tends to concentrate orotherwise occur (e.g., on the boom, just beyond the float), the testpoints in this example are located between the distal ends of the boomsand the attachment points where the floats and booms are connected, butcloser to the boom and further away from the outer rotors. As shownhere, in some embodiments, the (outer) rotor is attached to the boom ata distal end of the boom, a fuselage is attached to the boom at aproximal end of the boom, a float is attached to a boom at an attachmentpoint between the distal end and the proximal end, and there is a firsttest point and a second test point associated with injecting an injectedelectrical signal into the boom and receiving the received electricalsignal via the boom, respectively, wherein the first test point and thesecond test point are located between the distal end of the boom and theattachment point. As shown here, in some embodiments, the two testpoints are (more specifically) located between a midpoint (e.g., betweenthe boom/float attachment point and the distal end where the outer rotoris) and the boom/float attachment point. This may reduce an amount ofelectrical routing, which is desirable.

In some embodiments, if the thrust output by an outer rotor (350, 352,354, or 356) is reduced, the thrust output by one or more interiorrotors is increased in order to compensate for the reduced thrust fromthe outer rotor. Which rotors are increased to compensate for thedecreased thrust and by how much is implementation dependent and maydepend upon a variety of factors including geometry of the rotors,weighing of various metrics, etc. To put it (more) generally, inresponse to deciding to decrease the thrust which is output by an outerrotor, a second thrust (which is output by an inner rotor) is increasedwhere the inner rotor is located above the float.

Returning briefly to FIG. 2A and FIG. 2B, a variety of connectors may beused to physically connect the signal generator (204 a/204 b) and signalreceiver (206 a/206 b) to the boom (202 a/202 b) at their respectivetest sites. The following figures describe some exemplary connectors.

FIG. 4A is a diagram illustrating an embodiment of a connector at a testpoint which uses a screw inserted into the composite. In this example, atest site is shown at which an injected electrical signal is insertedinto the composite (400) or from which a received electrical signal isextracted from the composite (400). A cavity (402) is drilled into thelayers of the composite from the interior of the boom, without piercingthe exterior of the boom. Note, for example, that there is at least onelayer of composite left on the exterior-facing side of the composite.

In this example, a screw (404) is inserted into the cavity with an(electrical) connector (406) wrapped around the threads of the screw.Some examples of the connector (406) include copper wire. Although notshown here, in some embodiments, the connector is wrapped around thethreads of the screw within the cavity. In some embodiments, an adhesive(not shown) is applied over the components shown to keep the screw (404)and/or connector in their proper position (e.g., so that a connection isnot lost).

FIG. 4B is a diagram illustrating an embodiment of a connector at a testpoint which uses a connector placed next to exposed composite withadhesive applied over the connector. In this example, interior-facinglayers of the composite (420) have been sanded away so that the innerlayers of the composite are exposed. One or more connectors (422) areplaced against the exposed composite layers. As described above, in someembodiments, the connectors are copper wires. While the connector(s) aretouching the exposed composite layers, an adhesive (424) is applied overboth so that the connection is maintained.

FIG. 4C is a diagram illustrating an embodiment of a connector at a testpoint which uses a conductive layer. In this example, during formationof the composite (440), a conductive layer (442) is included as one ofthe inner layers of the composite. The conductive layer in this exampleincludes an exposed portion (444) which extrudes or extendssubstantially perpendicularly from the plane of the composite (layers),passing through any composite layers that are between the conductivelayer (442) and the interior of the boom. In various embodiments, theexposed portion may be substantially planar in shape (e.g., a conductiveflap or tab) or substantially cylindrical in shape (e.g., like a wire orcable).

Using the connectors shown in FIG. 4A-FIG. 4C, further physical and/orelectrical connections may be made. For example, it may be desirable tophysically secure the connector(s) against the interior of the boom sothat there is no pulling which could dislodge the connector(s) andreduce the quality of the (e.g., electrical and physical) connection. Insome embodiments, a connector (e.g., a copper wire) is wrapped orotherwise electrically insulated so that the connector is shielded fromnoise and/or will not “short” with another exposed wire. For example, inFIG. 4A, after being wrapped around the threads of the screw, connector406 may be wrapped or otherwise shielded going to the signal receiver orcoming from the signal generator.

It is noted that all of the connector embodiments shown here put theconnector(s) into electrical contact with the inner layers of thecomposite. For example, in FIG. 4A, the screw (404) may be an electricalconductor which puts the connector (406) into electrical contact withthe inner layers of composite. Alternatively, the connector may bewrapped around the threads of the screw and inserted into the cavity.Putting the connector(s) into electrical contact (e.g., directly and/orindirectly) with the inner layers of the composite may be desirablebecause (to use the received electrical signal as an example) it drawscharge or current from multiple layers of the composite (and thereforemay be a better and/or more representative sample). Exposing the innerlayers of the composite may also enable a more secure physicalconnection (e.g., so that none of the components shown become loose ordislodged).

Booms are often constructed of a composite material because it permits alightweight and hollow boom which has a good structural performance forits light weight. Some types of composite materials (e.g., carbon fiber)may have different electrical and/or physical properties depending uponthe state of the composite material (e.g., the temperature, the amountof water that has permeated the composite, etc.). The following figuresdescribe some examples of this and how to account for this when decidingwhether to decrease the thrust output by a rotor (e.g., at step 102 inFIG. 1).

FIG. 5 is a graph illustrating an embodiment of different responsefunctions depending upon the state of the boom. In the graph shown, twoexemplary response functions (500 and 502) are shown where the boom isin different states for each function. Each response function shows howa measured property (e.g., representative of the bending and/or strainexperienced by the boom) varies as the thrust increases. For example,the measured property may be a measured voltage or measured current if aDC signal is used across the boom. For convenience, linear functions areused for the response functions, but the response function may have anyshape.

Depending upon the state of the boom, the response function (i.e., howthe measured property, such as a voltage or current, responds to thethrust) can change. For example, suppose that response function 500 isassociated with a boom that has been in a wet or humid environment andresponse function 502 is associated with a boom that has been in a dryenvironment. Some types of composites use adhesives which absorb waterin wet or humid environments. The presence of water in the compositematerial can change the measured property (e.g., along the y-axis) inresponse to the thrust (e.g., the x-axis). For this reason, a wet boom(where water has permeated the composite material) may have a responsefunction like function 500 and a dry boom (where water has not permeatedthe composite material) may have a response function like function 502(as an example).

Some other examples of boom states which may affect the responsefunction include age (e.g., old composite may respond differentlycompared to new composite), temperature (e.g., composite that has beenin the sun or is in warmer ambient air may respond differently comparedto composite that has been in the shade or is in cooler ambient air),manufacturing differences, etc.

In general, the different states affect the electrical properties (e.g.,the resistance or impedance through the boom) and/or physical properties(e.g., the suppleness or malleability of the boom) so that a fixed orconstant value of a measured property is not always a good indication ofwhen to reduce a rotor's thrust. For example, suppose that theelectrical signal is a DC signal coming from a constant voltage sourceso that the current in the received electrical signal varies. If a fixedor static threshold of 1 Amps (as an example) is used to decide when toreduce the thrust, the thrust may be unnecessarily and/or prematurelyreduced when the boom is in some states and/or may be reduced too latewhen the boom is in other states.

Even if the amount of curvature or bending in a boom were directlymeasured (e.g., an angle of bending was measured), using a fixed angleof bending may not be a good measure of when the boom is experiencing“too much” bending or stress. For example, differences in temperature,moisture, and/or age may make the composite supple and pliable in onestate (and thus the boom can withstand more bending) versus anotherstate in which the composite is much more brittle and less pliable (andis less able to withstand bending

To account for this, calibration is performed in some embodiments.Generally speaking, calibration obtains data samples in order todetermine when to reduce the thrust produced by a rotor (i.e., when to“back off” a particular rotor because an attached boom is experiencingtoo much stress). In this example, calibration is performed each time anaircraft is turned on. For example, data sample 504 (with a thrust ofzero) may be obtained when the aircraft has turned on but before therotors have started rotating. As the aircraft begins to take off androtors output more thrust, additional data samples are obtained.

From the exemplary data samples (such as data sample 504), responsefunction 500 is a better fit for the data samples compared to responsefunction 502. In this example, once response function 500 is selected,the corresponding point 506 (which in this example is predefined) isused to decide when to reduce the thrust of a related rotor (at least inthis example). It is noted that response function 500 and responsefunction 502 have different threshold or trigger levels (e.g., 506 and508) which correspond to different measurement levels (e.g.,measurement₁ and measurement₂) and/or thrust levels (e.g., thrust₁ andthrust₂) at which to reduce a rotor's thrust.

In some embodiments, a finite number of predefined response functions(e.g., with corresponding (predefined) threshold levels) are stored in aflight controller. For example, predefined response functions may enableinformation obtained during bench testing (e.g., where the boom is undercontrolled temperature, moisture, and/or age conditions) and/ordestructive testing (e.g., given a boom at a controlled temperature,moisture, and/or age, what is the breaking point) to be used to generatethe predefined response functions.

Alternatively, in some embodiments, there are no predefined responsefunctions. For example, a polynomial function may be used to representthe response function and the calibration process selects thecoefficients of the polynomial function using the data samples.

The following figure describes the above calibration example moregenerally and/or formally in a flowchart.

FIG. 6 is a flowchart illustrating an embodiment of a process to limitthe thrust, if needed, of a boom-mounted rotor to prevent damage to theboom where calibration is performed. FIG. 6 is similar to FIG. 1 and forconvenience the same or similar reference numbers are used to showrelated steps.

At 100, an electrical signal is received via a boom, wherein a rotor isattached to the boom and the received electrical signal is receivedwhile the rotor outputs a thrust. See, for example, FIG. 2B.

At 600, calibration is performed, including by obtaining a plurality ofdata samples, wherein each data sample in the plurality of data samplesincludes an amount of the thrust output by the rotor and a measuredamount of a property that is used in analyzing the received electricalsignal in order to decide whether to decrease the thrust. In someembodiments, calibration is initiated each time an aircraft is turnedon. In some embodiments, calibration is initiated mid-flight.

In one example of the data samples which are collected by thecalibration process, suppose that the received electrical signal'scurrent is used to decide whether to decrease the thrust output by thecorresponding rotor. That is one example of a property that is used inanalyzing the received electrical signal in order to decide whether todecrease the thrust. Other properties which may be used include voltage,amplitude, phase, frequency, etc.

At 102′, it is decided whether to decrease the thrust which is output bythe rotor, including by analyzing the received electrical signal using athreshold amount of the property at which it is decided to decrease thethrust which is output by the rotor, wherein the threshold amount isbased at least in part on the plurality of data samples. To put itanother way, the data samples which are collected by the calibrationprocess are used to decide when the boom is experiencing too muchbending and/or stress and therefore the thrust output by the rotorshould be decreased.

At 104, in response to deciding to decrease the thrust which is outputby the rotor, decrease the thrust which is output by the rotor. See, forexample, FIG. 3B and Table 1 which describe which rotors would havetheir outputs reduced in the event a given boom is believed to have toomuch bending or stress.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A system comprising: a boom having a first testpoint at a first end of the boom and a second test point at a second endof the boom, wherein a rotor is attached to the boom; and a flightcontroller that: instructs a signal generator to inject an electricalsignal at the first test point; receives, in response to the injectedelectrical signal, an electrical signal at the second test point,wherein the received electrical signal is received while the rotoroutputs a thrust; decides to decrease the thrust which is output by therotor, including by analyzing the received electrical signal; and inresponse to deciding to decrease the thrust which is output by therotor, instruct the rotor to decrease the thrust.
 2. The system recitedin claim 1, wherein the flight controller: decides to increase thethrust which is output by the rotor, including by analyzing the receivedelectrical signal; and in response to deciding to increase the thrustwhich is output by the rotor, instructing the rotor to increase thethrust.
 3. The system recited in claim 1, wherein: the rotor is attachedto the boom at a distal end of the boom; a fuselage is attached to theboom at a proximal end of the boom; a float is attached to the boom atan attachment point between the distal end and the proximal end; and thefirst test point and the second test point are located between thedistal end of the boom and the attachment point.
 4. The system recitedin claim 1, wherein: the rotor is an outer rotor which is attached tothe boom at a distal end of the boom; a fuselage is attached to the boomat a proximal end of the boom; a float is attached to the boom at anattachment point between the distal end and the proximal end; and inresponse to deciding to decrease the thrust which is output by the outerrotor, a second thrust which is output by an inner rotor is increased,wherein the inner rotor is located above the float.
 5. The systemrecited in claim 1, wherein the received electrical signal is receivedusing an electrical connector that is wrapped around one or more threadsof a screw which is inserted into the boom from an interior of the boomwithout piercing an exterior of the boom.
 6. The system recited in claim1, wherein the received electrical signal is received using anelectrical connector that is held in contact with an inner layer ofcomposite in the boom using an adhesive.
 7. The system recited in claim1, wherein: the boom includes a plurality of composite layers; anelectrically conductive layer is an inner layer of composite in theplurality of composite layers; and the electrically conductive layerincludes an exposed portion which extends into a hollow interior of theboom.
 8. The system recited in claim 1, wherein: the flight controllerperforms calibration, including by obtaining a plurality of datasamples, wherein each data sample in the plurality of data samplesincludes an amount of the thrust output by the rotor and a measuredamount of a property that is used in analyzing the received electricalsignal in order to decide to decrease the thrust; and analyzing thereceived electrical signal further includes using a threshold amount ofthe property at which it is decided to decrease the thrust which isoutput by the rotor, wherein the threshold amount is based at least inpart on the plurality of data samples.
 9. The system recited in claim 8,wherein the property represents at least one of bending and strainexperienced by the boom.
 10. The system recited in claim 8, wherein theinjected and received electrical signal is a DC signal and the propertyincludes at least one of voltage and current.
 11. The system recited inclaim 8, wherein the injected and received electrical signal is an ACsignal and the property includes at least one of phase, frequency, andamplitude.
 12. A method comprising: instructing a signal generator toinject an electrical signal at a first test point, wherein the firsttest point is at a first end of a boom and the boom has a second testpoint at a second end and a rotor is attached to the boom; receiving, inresponse to the injected electrical signal, an electrical signal at thesecond test point, wherein the received electrical signal is receivedwhile the rotor outputs a thrust; deciding to decrease the thrust whichis output by the rotor, including by analyzing the received electricalsignal; and in response to deciding to decrease the thrust which isoutput by the rotor, instructing the rotor to decrease the thrust. 13.The method recited in claim 12, further comprising: deciding to increasethe thrust which is output by the rotor, including by analyzing thereceived electrical signal; and in response to deciding to increase thethrust which is output by the rotor, instructing the rotor to increasethe thrust.
 14. The method recited in claim 12, wherein: the rotor isattached to the boom at a distal end of the boom; a fuselage is attachedto the boom at a proximal end of the boom; a float is attached to theboom at an attachment point between the distal end and the proximal end;and the first test point and the second test point are located betweenthe distal end of the boom and the attachment point.
 15. The methodrecited in claim 12, wherein: the rotor is an outer rotor which isattached to the boom at a distal end of the boom; a fuselage is attachedto the boom at a proximal end of the boom; a float is attached to theboom at an attachment point between the distal end and the proximal end;and in response to deciding to decrease the thrust which is output bythe outer rotor, a second thrust which is output by an inner rotor isincreased, wherein the inner rotor is located above the float.
 16. Themethod recited in claim 12, wherein the received electrical signal isreceived using an electrical connector that is wrapped around one ormore threads of a screw which is inserted into the boom from an interiorof the boom without piercing an exterior of the boom.
 17. The methodrecited in claim 12, wherein the received electrical signal is receivedusing an electrical connector that is held in contact with an innerlayer of composite in the boom using an adhesive.
 18. The method recitedin claim 12, wherein: the boom includes a plurality of composite layers;an electrically conductive layer is an inner layer of composite in theplurality of composite layers; and the electrically conductive layerincludes an exposed portion which extends into a hollow interior of theboom.
 19. The method recited in claim 12, wherein: the method furtherincludes performing calibration, including by obtaining a plurality ofdata samples, wherein each data sample in the plurality of data samplesincludes an amount of the thrust output by the rotor and a measuredamount of a property that is used in analyzing the received electricalsignal in order to decide to decrease the thrust; and analyzing thereceived electrical signal further includes using a threshold amount ofthe property at which it is decided to decrease the thrust which isoutput by the rotor, wherein the threshold amount is based at least inpart on the plurality of data samples.
 20. A computer program productembodied in a non-transitory computer readable storage medium andcomprising computer instructions for: instructing a signal generator toinject an electrical signal at a first test point, wherein the firsttest point is at a first end of a boom and the boom has a second testpoint at a second end and a rotor is attached to the boom; receiving, inresponse to the injected electrical signal, an electrical signal at thesecond test point, wherein the received electrical signal is receivedwhile the rotor outputs a thrust; deciding to decrease the thrust whichis output by the rotor, including by analyzing the received electricalsignal; and in response to deciding to decrease the thrust which isoutput by the rotor, instructing the rotor to decrease the thrust.